A printed wiring board has a substrate made of an insulating resin such as an epoxy resin, and a copper foil that has been etched to have a desired pattern, which can be produced by, for example, a subtractive process. In the usual substractive process, two copper foils are laminated on both surfaces of a prepreg by hot pressing to form a copper-clad laminate having the cured prepreg as a substrate. Holes are opened in the copper-clad laminate which is then electroless plated, followed by electroplating to electrically connect the copper foils on both surfaces of the substrate. The copper foil surfaces are coated with a photoresist, and then exposed to UV light so as to produce a desired resist pattern. Subsequently, the copper is etched by an acid or an alkali etchant to form the desired wiring pattern. On the printed wiring board having the wiring pattern, electronic devices and/or elements, etc. are mounted to obtain a printed circuit board.
To enhance the bond strength between the substrate and the copper foil, the surface of the copper foil for a printed wiring board is generally treated by various chemical or electrochemical techniques, for example, a bond enhancing treatment (so-called "burning plating" treatment) by which a particulate copper deposit, e.g. a nodular copper deposit (fine cluster-like deposit) or a whiskery copper deposit is formed on the surface of the copper foil. Further, a chromate layer is formed on the surface of the copper foil to prevent deterioration of the bond strength between the copper wiring pattern and the substrate caused by undermining in the etching process by an acid etching solution or an alkali etching solution. Further, a zinc plated layer is also plated on the surface of the copper foil in order to enhance the heat resistance of the laminate.
With recent progress of globalization of the copper foils market, exportation of copper foils has increased, so that the copper foils may be stored under various conditions for a long time.
As materials of the substrate, various resins have been used, and not only epoxy resin substrates hitherto widely used but also other resins which are made from a varnish containing an organic acid. For example, recently, a copper-clad laminate is produced by continuously laminating copper foils on both sides of a fibrous base material impregnated with a varnish for an acrylic resin, which contains an acrylic acid, and curing the varnish. In such a case, the copper foil necessarily contacts the organic acid contained, in the varnish under the curing conditions.
The printed wiring boards occasionally undergo heat processing such as soldering and solder resist curing when electronic devices are mounted thereon. Further, in practice, printed circuit boards are occasionally placed in high temperature environments, such as in the engines room of automobile, for a long time.
Accordingly, copper foils having excellent chemical resistance and heat resistance have been desired.
Among conventional copper foils having been subjected to bond enhancing treatments, the electroplating of zinc and the chromate treatment, however, there has been found no copper foil excellent in both the chemical resistance and the heat resistance.
For example, there has been conventionally used for preparing a printed wiring board a copper foil having 31 to 600 mg/m.sup.2 of an electroplated zinc, 10 to 100 mg/m.sup.2 (in terms of arsenic atom) of an arsenical copper and 1 to 20 mg/m.sup.2 (in terms of chromium) of a plated chromate and further provided with a silane coupling agent. But when the copper foil contacts a varnish containing organic acid, for example, an acrylic resin, in the formation of a copper-clad laminate, the bond strength between the copper foil and the substrate is insufficient. Further, even if the copper foil is laminated to a substrate mainly made of an epoxy resin, the bond strength between the copper foil and the substrate deteriorates by contact with an acid solution or an alkali solution during or after the etching process.
In addition, there has been conventionally used another copper foil having 1 to 30 mg/m.sup.2 of a plated zinc and 1 to 20 mg/m.sup.2 (in terms of chromium) of a chromate. However, when the copper foil contacts varnish containing organic acid in the formation of the copper-clad laminate, the acid attacks the zinc layer. As a result, sufficient bond strength between the substrate and the copper foil cannot be obtained. The above-described problems are caused by extremely poor chemical resistance of the conventional copper foils.
Further, when the copper foils as described above are laminated with a substrate mainly made of a brominated epoxy resin, the heat resistance is insufficient, so that the bond strength between the copper circuit and the substrate will deteriorate during the long-term heating when the printed circuit board is placed in an engine room of an automobile, and finally the copper circuit may blister from the substrate.
In Japanese Patent Laid-Open Publication No. 231161/1995, there is proposed, as a copper foil of excellent heat resistance, a copper foil having a ternary alloy layer of copper-zinc-tin or copper-zinc-nickel and further having a chromate layer on the surface of the alloy layer. However, the copper foil is not always satisfactory in acid resistance after long-term storage. The reason for the unsatisfactory acid resistance is presumably, as follows, that, during the long-term storage, zinc excessively diffuses from the ternary alloy layer such as copper-zinc-tin alloy layer or copper-zinc-nickel alloy layer to the copper foil. Simultaneously copper diffuse from copper foil to the alloy layer. Consequently, resistance of the alloy layer against chemicals is weakened. Further, when the copper foil described above is contacted with a varnish containing an organic acid, such as a varnish for acrylic resin, which contains acrylic acid, in the formation of the copper-clad laminate, the interface between the substrate and the copper foil may be attacked by the acid before and during curing of the varnish. On this account, it is very difficult to keep sufficient bond strength between the substrate and the copper foil and, as a result, properties of the resulting printed wiring board are also insufficient.